1. Field of the Invention
This invention relates generally to the controlled cooling of hot rolled steel products such as rods and the like in direct sequence with the rolling operation in order to achieve predetermined metallurgical qualities
2. Description of the Prior Art
The controlled cooling of hot rolled steel rod in direct sequence with the rolling thereof began approximately twenty years ago with the process described in U.S. Pat. No. 3,231,432 (McLean et al). This process involves hot-rolling the rod and thereafter directly coiling it onto an open conveyor in spread out ring form while the microstructure of the steel is still in a condition of highly uniform, relatively small austenite grain size. While moving along the conveyor, the rings are air cooled through allotropic transformation. This produces a "patented" microstructure, i.e., a microstructure sufficiently equivalent to that achieved by air or lead patenting so as to enable the rod to be subsequently processed to a finished product, as for example by being drawn into wire, without additional heat treatment.
In the earlier installations of this process, chain-type conveyors were employed. However, because of the tendency of the rings to undergo scratching as a result of their being dragged over stationary support rails located between the chains, and because of the non-uniform cooling which results from prolonged area contact with such rails, the use of chain-type conveyors has of late been largely discontinued in favor of roller conveyors of the type shown for example in U.S. Pat. No. 3,930,900 (Wilson). Here, the rings are transported over driven rollers, with air nozzles arranged between the rollers to blow cooling air upwardly through the rings.
Further improvements in uniformity of cooling and flexibility of operation have been achieved by arranging the air cooling nozzles directly under the conveyor rollers, as shown for example in U.S. Pat. No. 4,448,401 (Jalil et al).
The rod sizes that are processed in installations of the foregoing type typically range from 5-19 mm. in diameter For rods below about 9 mm. in diameter, air cooling has proven to be fast enough to achieve acceptable tensile strengths. However, for rods 9 mm. and above in diameter, air cooling rates are not sufficiently rapid, thus yielding tensile strengths which are below acceptable levels for certain applications.
Attempts have been made at achieving increased cooling rates by employing water as a cooling medium. See for example U.S. Pat. No. 4,395,022 (Paulus et al) which describes an apparatus for cooling hot rolled steel products, including rod, by immersion in a water bath. Cooling by water immersion has reportedly achieved somewhat accelerated cooling rates with improved tensiles for larger rod sizes. However, uniform results have been difficult to achieve, primarily because of the difficulty of maintaining optimum water chemistry. This problem is compounded by the need to continuously remove contaminants such as dirt, mill scale, etc. from the water bath.
As shown by U.S. Pat. No. 4,168,993 (Wilson et al), some work has been done with water sprays. The problem with these arrangements, however, is that certain conveyor zones are limited to cooling by a water spray application, whereas other conveyor zones are limited to cooling by air. If only air cooling is required, as for example when processing smaller diameter rods, then the water cooling zones must be shut down. Thus, as the rod moves along the conveyor, it experiences extended intervals (while moving through the inoperative water cooling zones) when no coolant is being applied. This seriously compromises the overall effectiveness of such processes.
Because of the foregoing problems, when rolling rod diameters of 9 mm. and above, most mills either draw wire to greater reductions, or use alloying elements to increase the hardenability of the steel, or resort to off-line lead or salt patenting heat treatments. The first of these alternatives yields mixed results, and the second and third alternatives significantly increase tonnage costs. In short, the prior art has failed to satisfactorily meet the demands of the industry when processing the larger rod sizes ranging from 9-19 mm. in diameter.